One approach to cancer treatment is external beam radiotherapy or radiation therapy involving the treatment with a high intensity electron, photon, proton or hadron beam. In Image Guided Radiation Therapy (IGRT) the goal of maximizing the dose delivered to the tumor while minimizing the dose to surrounding tissue is enabled by information extracted from different imaging modalities and fed into the treatment planning and verification workflow.
Typically, more than 50% of all newly diagnosed cancer patients require external beam therapy as the main component of their treatment. Conventional external radiotherapy relies on radiation generated with a medical linear particle accelerator (LINAC) which, by means of a high intensity electron beam, produces either electron or photon treatment beams. Advances in linear accelerator techniques and controls have allowed the emergence of new treatment strategies like, e.g., Intensity-Modulated Radiation Therapy (IMRT) or volumetric-modulated arc therapy (VMAT), designed to maximize tumor control and to minimize damage to critical areas and healthy tissues. The potential clinical gains offered by the new treatment approaches can, however, be impaired by insufficient patient immobilization, set-up errors and verification at the time of treatment delivery. Occurrence of motion or tumor migration in relation to surrounding tissue make it necessary to include a step for treatment verification and treatment planning right before treatment.
In Image Guided Radiation Therapy with conventional photon beams two main techniques for beam monitoring and patient positioning have been introduced: megavoltage (MV) imaging using the same or a modified treatment beam (i.e. the radiation generated by the LINAC) or kilovoltage (kV) imaging using an additional X-ray beam. The LINAC energy spectrum ranges from the kilovoltage up to the megavoltage energy regime (end-point region around 6-18 MeV). Although most of the photons are in fact of energy less than 1 MeV, usually the terminology “megavoltage radiation” is used when referring to this type of radiation. In the context of this application, megavoltage X-ray radiation thus refers to energies in the kV and MV regime.
For many years megavoltage imaging using electronic portal devices (EPDI), either based on film or active matrix flat panel imagers (AMFPIs), was traditionally the main technique for geometric verification of field placement as well as dosimetric validation. However, verifying the position of a soft tissue target volume with megavoltage energy photons is challenging due to very small differences in the photon attenuation coefficient at these energies. In parallel to conventional 2D imaging, techniques for 3D imaging based on Cone Beam CT (CBCT) were also implemented. In the past solutions to improve the detection efficiency of AMFPIs by means of segmented heavy scintillator panels either in parallel of focused configurations have been proposed. However, these solutions usually come at the expense of degraded spatial resolution due to beam divergence.
As an alternative to MV electronic portal imaging, on-board kV imaging recently emerged as a way to improve the contrast of soft tissue and bones and is currently one of the main options for IGRT due to the prospect of enhanced localization of target volumes and adjacent organs at risk. Most commercial implementations rely on a kV X-ray source and an opposing amorphous silicon flat panel imager, mounted at 90° to the treatment head for the acquisition of kV X-ray projection images for radiography and fluoroscopy. The dose from a kV CBCT is lower than the dose from wide-field MV portal imaging, especially for anatomical sites where contrast is usually low requiring additional exposure time with MV beams. One example for such a detector is used in the Elekta Synergy system and relies on a Gd2O2S:Tb (GOS) screen readout by an amorphous silicon flat panel (RID 1604, Perkin-Elmer Optoelectronics). It has an active area of 41×41 cm2 addressed as an array of 1024×1024 pixels, 400 micrometer pitch. The panel is located about half a meter from the axis of rotation and images are captured at a fixed frame rate of 2.7 Hz. Newer systems are being installed with a CsI panel, offering improved data acquisition at a higher frame rate.
In the Siemens Healthcare White Paper: “In-Line kView Imaging—The new standard in Image-Guided Radiation Therapy” an IGRT approach is presented which makes use of a modified treatment beam for imaging. The existing treatment beam is modified and optimized for imaging in order to provide kV-like images with high 2D and 3D soft tissue contrast using the on-board MV AMFPI device mounted on the LINAC gantry. For treatment purposes a flattening filter is used to achieve uniform dose across the field. This flattening filter absorbs low-energy photons, which are essential for high-contrast images. By removing this flattening filter for imaging purposes, the approach allows to utilize exactly those low-energy components in the beam. A beam of around 4 MV is typically used. In addition, the special carbon target used for imaging further shifts the energy spectrum toward the kV range which is more suitable for imaging. The modified beam can be detected with a regular flat panel detector. However, it is required to modify the beam for imaging.
In WO 2010/057500 A1 a radiation detector with doped optical guides is disclosed. The detector is suitable for use in connection with particle therapy applications and comprises at least one set of scintillating optical guides which upon exposure to incident radiation generate scintillating light. The optical guides are arranged in an array, such as in a so-called harp configuration, for detecting a transversal radiation beam profile.
In US 2012/0292517 A1 a radiation therapy system including a linear accelerator configured to emit a beam of radiation and a dosimeter configured to detect in real-time the beam of radiation emitted by the linear accelerator are disclosed. The dosimeter includes at least one linear array of scintillating fibers configured to capture radiation from the beam at a plurality of independent angular orientations, and a detection system coupled to the at least one linear array, the detection system configured to detect the beam of radiation by measuring an output of the scintillating fibers.
In US 2012/0205530 A1 a fluence monitoring detector for use with a multileaf collimator on a radiotherapy machine having an x-ray radiation source is disclosed. The fluence monitoring detector includes a plurality of scintillating optical fibers, a plurality of collection optical fibers coupled to the opposing ends of the scintillating optical fibers and a photo-detector coupled to the collection optical fibers.
In US 2007/0164225 A1 a Cerenkov x-ray detector for portal imaging is disclosed. The detector includes an optical-fiber taper (OFT) made from a large number of optical fibers. The optical-fiber taper is a matrix of optical fibers with the core material made of, e.g., silica and coated with a cladding glass or polymer. Each optical fiber in the optical fiber taper is fully aligned with the incident x-ray source so that x-rays entering the top of the fiber travel directly towards the bottom of the same fiber.